In resonant converter, a current flowing through power switches is sinusoidal wave per a resonant tank is employed. As a result, zero-voltage switching (ZVS) or zero-current switching (ZCS) is easy to be achieved.
Take a half-bridge LLC resonant converter as an example, a traditional method to achieve ZVS and ZCS is to design an appropriate switching frequency and associated dead-time period, so that when switched ON, there is no voltage across power switches, typically MOSFETs, and a current flowing through the power switches is negative. However, the half-bridge LLC resonant converter may work at a capacitive mode which is dangerous if the dead-time period is designed too long, and ZVS may be lost with a load current decreasing if the dead-time period is designed too short.